1. Field of the Invention
The present invention relates to a fuel cell system including a hydrogen concentration reduction process unit for reducing the concentration of hydrogen discharged from a fuel cell.
2. Description of the Related Art
Among fuel cells that are installed on fuel cell vehicles, there is a fuel cell of a type in which an anode and a cathode are provided on both sides of a solid polymer electrolyte membrane, respectively, whereby a fuel gas (for example, a hydrogen gas) is supplied to the anode and an oxidant gas (for example, oxygen or air) is supplied to the cathode, so that chemical energy resulting from oxidation/reduction reactions of these gases is extracted directly as electric energy.
In this type of fuel cell, in general, in order to improve the fuel consumption by increasing the fuel utilization factor, unconverted hydrogen which is discharged from the fuel cell without being consumed is recovered to be mixed with fresh fuel gas for re-supply to the fuel cell.
In addition, in this fuel cell, in order to maintain the electricity generation in a good condition by preventing the solid polymer electrolyte membrane from being dried, water is supplied to the reaction gas (either or both of hydrogen gas and oxidant gas). Furthermore, in this type of fuel cell, water is formed in conjunction with generation of electricity. This sometimes causes water to collect on the anode side (flooding), and in the event that water so collects, the supply of hydrogen to the anode is interrupted, leading to unstable generation of electricity.
In addition, since, although it may be minute in quantity, nitrogen contained in air supplied to the cathode conducts through the solid polymer electrolyte membrane to the anode side to be mixed with hydrogen, in the event that the concentration of nitrogen increases due to reuse of hydrogen gas, there may be caused a case where the generation of electricity becomes unstable.
In the related art, as a recovery method for dealing with such an unstable generation of electricity, there exists a method for continuing the stable generation of electricity by discharging gas from a hydrogen gas circulating flow path so as to discharge water that has collected at the anode and nitrogen mixed in hydrogen gas (for example, refer to JP-A-2002-93438). However, since hydrogen is also discharged when gas is discharged from the hydrogen gas circulating flow path, it is not good to discharge the gas into the atmosphere as it is.
Then, to cope with this, it is under consideration to mix gas (hydrogen) so discharged with air discharged from the cathode (hereinafter, referred to as discharged air) so as to reduce the hydrogen concentration before the gas is discharged into the atmosphere. A device used to reduce the hydrogen concentration like this is a hydrogen concentration reduction device.
In the fuel cell system having the hydrogen concentration reduction device, the discharge of gas from the hydrogen gas circulating flow path is controlled such that the instantaneous value of hydrogen concentration of gas discharged from the hydrogen concentration reduction device becomes equal to or smaller than a predetermined threshold by detecting the instantaneous value of hydrogen concentration of the gas with a hydrogen sensor.
However, even if the instantaneous value of hydrogen concentration of gas discharged from the hydrogen concentration reduction device is equal to or smaller than the predetermined threshold, since the quantity of hydrogen that has been so discharged is increased when a condition continues for a long time in which the instantaneous value of hydrogen concentration is close to the threshold, it is not preferable to discharge gas from the hydrogen gas circulating flow path.